The present invention relates to an apparatus and method for determining with accuracy the liquid water content of ambient air, particularly in relation to air flows across air vehicles or other structures. The accurate and timely measurement of liquid water content (LWC) permits prompt signalling for activating deicing systems, and also permits sensing atmospheric conditions for reporting or research purposes.
Unheated bodies exposed to airflow laden with supercooled water droplets will typically accrete ice as the droplets impact the body and freeze. Icing is particularly a problem with air vehicles. Determining when ice is starting to form or predicting when it will form is important in aircraft management of deicing equipment including heaters, which can consume huge amounts of power. When the air temperature is cold enough, 100% of the droplets carried in the airflow will freeze. If the temperature warms or airflow is increased, the energy balance relationship is altered. A critical liquid water content is reached where not all of the impinging supercooled water droplets freeze. This critical liquid water content is defined as the Ludlam Limit. The Ludlam Limit is described in an article by F. H. Ludlam entitled The Heat Economy of a Rimed Cylinder. Quart. J. Roy. Met. Soc., Vol. 77, 1951, pp. 663-666. Additional descriptions of the problem are in articles by B. L. Messinger, entitled Equilibrium Temperature of an Unheated Icing Surface as a Function of Air Speed, Journal of the Aeronautical Sciences, January 1953, and a further article entitled An Appraisal of The Single Rotating Cylinder Method of Liquid Water Content Measurement, by J. R. Stallbrass, Reportxe2x80x94Low Temperature Laboratory No. LTR-LT-92, National Research Council, Canada, 1978.
It has been shown that if the LWC increases above the Ludlam Limit, the accretion characteristics in theory remain unchanged, because excess water simply blows off or runs off, rather than freezing. Thus, present systems for determining liquid water content based on ice accretion suffer degraded accuracy above the Ludlam Limit. The Ludlam Limit for a given temperature and airflow is the liquid water content above which not all of the water freezes on impact with an accreting surface.
Accretion based ice detectors are frequently designed with probes that permit ice build up to a set mass, perhaps taking 30 to 60 seconds depending on conditions, at which time the presence of ice is enunciated or indicated, and a probe heater energized to melt the ice. Such ice detectors are well known in the art, and many depend upon a vibrating sensor or probe, with a frequency sensitive circuit set to determine frequency changes caused by ice accreting on the detector probe.
Liquid water content can be roughly determined by monitoring a signal proportional to the probe icing rate, which again can be determined with existing circuitry, but accuracy degrades rapidly if the liquid water content is above the Ludlam Limit, because a portion of the impinging water never freezes. In such cases the actual liquid water content will be under reported, with the Ludlam Limit liquid water content being the maximum that will be reported. Even though the droplet cloud may contain additional liquid water, there will be no indication from such an ice detector that there is additional liquid water in the air flow. Thus, the prior art devices will not discern the actual liquid water content when the Ludlam Limit has been exceeded.
The present invention relates to determining the liquid water content in an airflow, in particular, air flow past an air data sensing probe on an air vehicle. The amount of the liquid water in the airflow is determined even for liquid water content levels above the Ludlam Limit. The present invention senses ice growth rate on a vibrating probe type ice detector. The ice growth rate is predictably variable over an accretion cycle based upon the incremental rate of change of the vibrating probe""s frequency throughout the sensing cycle. The rate of change of probe vibration frequency (df/dt) throughout the ice accretion cycle is determined. Further, the rate of frequency change (df/dt) characteristics are demonstrated to be a predictable function of liquid water, content, even above the Ludlam Limit, meaning that liquid water content can be determined at the higher liquid water content level.
The rate of change of probe vibrating frequency is determined for all or a portion of the ice accretion phase of the probe operating cycle, because it has been determined that this rate of frequency change (df/dt) is a function of liquid water content at that time.
In order to measure liquid water content with the present invention, the air speed and the temperature of the ambient air must be known. These basic parameters are readily available from an air data computer, using outside instrumentation, such as a pitot tube or a pitot-static tube, and a temperature sensor, such as a total air temperature sensor. The known liquid water content at a particular known air speed, temperature and rate of change of the vibration frequency of a vibrating probe ice detector are determined and combined in a look up table. The values can be determined by actual icing wind tunnel tests, or test results can be used to derive an algorithm that provides liquid water content when the three variables, air flow rate (or air speed), temperature and rate of change of frequency of vibration caused by ice accretion are known. Although a frequency rate of change is described herein, the rate of change of other signals sensitive to ice accretion could be used. A signal based on the rate of change of ice accretion (but not merely the amount of ice accretion) is a key to proper results.
The overall accretion time has been found to decrease with increasing liquid water content in most cases, but this is not assured. This invention is dependent on ice accretion, and will approach some limit of usefulness when operating conditions are such that little or no ice accretes on the probe. This may occur under conditions of warmer air temperature and high aerodynamic heating, for example.